PostScript is a resolution-independent document format. PostScript fonts can be enlarged or reduced in size to accommodate any viewing resolution. When a viewer zooms in or out of a PostScript document, the text characters automatically scale accordingly. Thus it can be said that PostScript font characters are “scalable.”
Similarly, graphical objects based on vector graphics consisting of line segments and curves are also scalable. The line segments and curves can be enlarged or reduced in size by appropriately modifying the pixel coordinates of their control points.
Raster graphics, on the other hand, is not scalable. An image expressed in raster graphics is by its nature pixel resolution specific, and to enlarge or reduce the image involves digital image filtering and interpolation. Moreover, a raster image cannot be stretched beyond its original pixel resolution without introducing additional color data, such as interpolated color data, to the original color data.
In the field of pre-press graphic arts, routinely processed documents contain high resolution raster graphics, with quality levels at or near photographic quality. Such documents are typically stored electronically as very large files, the large size being due primarily to the raster images therewithin. As a result, such documents are unwieldy to process and to transmit. Traditionally pre-press service providers prepared sample proofs of jobs for their customers, and the customers came to the company shops in person to inspect the proofs, before the print productions were run. Today, however, many pre-press service providers use the Internet as a way to transfer jobs to customers for proofing. This has many advantages. The customer does not have to come to the shop in person. The customer does not have to inspect his job and mark his changes “on the spot.” The customer can show his proof to others, and solicit their feedback. Proofs can be sent back and forth between the shop and the customer more often than before.
Use of the Internet for proofing has its shortcomings, though. On account of the large sizes of the files involved, when a customer proofs an electronic pre-press job on-line, there are unacceptable delays in interactively viewing the job, and in transmitting the job back and forth between the pre-press computer and a customer computer.
These delays can be mitigated somewhat by using only low resolution images in the document that is transmitted for proofing, but this is done at the expense of lost quality. A customer proofing a job with low resolution images is not able to inspect the quality of the images—which is one of the main objectives of proofing.